The subject matter discussed in the background section should not be assumed to be prior art merely as a result of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may also be inventions.
In the field of image display and projection devices, including liquid crystal displays (LCD), gas plasma display panels (PDP's), organic light-emitting diode (OLED), DMD (digital micro-mirror devices), and cathode ray tube (CRT), among the leading and most successful technologies, artificial limitations exist today which prevent the further development of many performance and value criteria and desirable new display features for devices based on these (or any) core modulation technologies.
A major artificial limitation on the further development of any display or projection modulation technology is the tendency to conceive of any display technology as identical to the modulation technology employed to change the fundamental state of pixel or sub-pixel “on” (lighted) or “off” (dark). A display technology is generally thought of as identical to the pixel-state modulation technology itself. Thus, in general, improvements of the display technology is conceived of as improving the characteristics of an integrated modulator device, the “light-valve.”
Focus has therefore been on improving such modulator device features as the color transmission efficiency of the modulator materials for each color of whatever color system (typically, red-green-blue or RGB) is employed to realize color in a display; related thermal efficiency of the modulator device for the colors which pass through the modulator; switching speed of the modulator device for the colors which pass through the modulator; power consumption of the integrated color modulator; filtering efficiency of modulators which modulate white light and which must be color-filtered; and spatial compactness of the device, especially in the viewing plane (for minimum fill-factor between sub-pixels or pixels), but also in the depth of the device for direct-view displays where thinness is desired. Flexibility of the display structure is also desirable for many applications, and there are limitations on options to achieve this when there is an assumption of one integrated modulator device per sub-pixel.
Solutions to this conceptual straight-jacket have been proposed by the inventor of the present disclosure (the incorporated pending '461 application), which de-couples the conception of an image display or projection system from this one-integrated-modulator-per-viewable-sub-pixel constraint. Passive and active optical components, including novel 3-dimensional textile structures employing novel passive and active optical fibers, are proposed as part of a novel telecom-structured display architecture, in which signal is generated, distributed, and aggregated from modulation means to realize many-to-one and one-to-many relationships between sub-pixel signal generation and final viewable sub-pixels and pixels.
A co-pending disclosure by the inventor of the present disclosure, the incorporated '361 application, applies those principles generally to the problem of pixel modulation itself at the basic constituent level, with a very wide range of embodiments that follow and are encompassed thereby.
A further application of this approach to the problem of an improved hybrid telecom-type display system is detailed in the present disclosure, combining techniques of “classical” and hybrid magneto-optic or magneto-photonic pixel logic with frequency modulation processes, such as phosphor absorption-emission systems and/or periodically-poled materials such as Ti:PPLN, PPKTP, and PPLN, and shocked crystals, band-optimization, or polarization mode processing stages, and optional signal amplification/gain signal amplification/gain stages, devices, or materials—to achieve display systems of substantially improved performance in all characteristics as compared to any other conventional display system solution. In addition, configurations of systems contemplated by the present disclosure enable local network and long-distance adaptation of and implementation for DWDM-type systems.
In summary, the present disclosure details both novel and improved systems as well as certain novel and improved components which may be used by such systems.
At this point, it is essential to review the history of the development of “magneto-optic/photonic” displays in general. After this review and consideration of the problems and solutions that have been developed up to this point, it will also become apparent what limitations still exist to further progress caused by a “mono-technology” approach to this category of display system.
The present disclosure thus includes not only the proposal of a system employing MO/MPC as a stage in “pixel-signal processing” and thus as a component technology of an overall telecom-type display system that is superior thereby, but it also proposes novel MO/MPC modulation technologies (devices and components and materials structures and systems) to realize improvements from this component contribution source as well.
However, in many if not all embodiments of the novel MO/MPC system, it is not required that the MO/MPC improvements be realized as a part of a telecom-type or structured system, but rather there is that much more performance and benefit that is realized by implementation of the improved component in the division-of-labor system herein proposed and in the previously referenced general disclosure of the co-pending application.
MO/MPC Display Development: A Brief History
Starting in the 1960's, the first three decades (approximately) of attempts to achieve a practical magneto-optic display saw many initial problems confronted, with some addressed partially at least, but no achievement of a practical display employing a light-valve based on a magneto-optic effect.
The first proposal for a display based on a magneto-optic light valve was made in the late 1960's, specifically, a proposal for a display formed from a passively-addressed array of discrete solenoid-type classical Faraday-effect rotators (relatively-high Verdet-constant garnet crystals wound with wire, similar to core memory elements), combined with crossed polarizers to realize a light-valve structure similar to an LCD light-valve. (Patent reference GB1,180,334, hereby incorporated by reference.)
However, the most significant effort was the one undertaken at Litton Industries, beginning in the eighties and continuing into the nineties, again evidently inspired by techniques from magnetic memory technology development—in this case, bubble memory.
In this effort, employing thick iron garnet films, a few of the basic system requirements that might be assumed to be required for a practical MO display were addressed.
Active-matrix addressing, an obvious implementation of technology developed for LCD and other array technologies (displays and sensors), magnetic domain management techniques, and increasing the B field by means of increasing the number of windings of a superficial coil-like structure were among the significant solutions proposed.
Reflecting the essential limitations of the Litton bubble-memory-type approach was this strategy for reducing power and potentially increasing switching speed by means of this coil-structure proposal. Specifically, the proposal was to deposit sinuous, recursive loops on the surface of the MO films surrounding each subpixel, with the lines forming a continuous series of small loops as the conductive track eventually “encircling” the subpixel in one continuous track of horizontal “coil windings”, the line never “crossing” itself, before returning to the origin. In appearance these looked like “squiggles.”
This coil-winding-like, superficial field-generating structure, realizing a coil-like additive effect on the net B field surrounding the subpixel, both reflected other problems unaddressed by Litton, as well as creating or compounding others.
Without knowing anything about the optical or magnetic problems with the Litton designs, it is obvious that the strategy of increasing B field and domain switching efficiency by using the surface of the crystal surrounding the relative “core” for a superficial winding structure would negatively impact display performance by increasing fill-factor.
In addition, this strategy did not address and actually would exacerbate the problem of magnetic cross-talk between subpixels. Because of the employment of continuous bulk MO films as the deposition surface for field-generating “squiggles,” the increase in B field without any structure and/or materials strategy to isolate each sub-pixel from the other will result in increased cross-talk and thus a degradation of the contrast and in fact the basic operation of the array as a display.
Magnetic cross-talk is a problem in the Litton design because adjacent sub-pixels are partially or almost fully switched; from the point of view of the viewer of the display, the array becomes a smudged blur. (It should be noted that the problem of magnetic cross-talk also existed for other and earlier attempts at a practical magneto-optic display).
Optical cross-talk was another fundamental problem of the Litton approach, as it had also been for earlier attempts.
What was missing from all prior attempts at a practical magneto-optic display, whether transmissive or reflective (both modes being possible because of the non-reciprocal nature of the classical Faraday Effect of magnetic-field induced polarization rotation), was a waveguiding structure to effectively form a pixel, or what is referred to in the present disclosure as a “pixel signal.”
This absence is evident from the fact that, within the continuous MO film, there was no structure to control the path of the incident beam to ensure its passage only through a designated modulation zone and not through the spaces between pixels or into other pixels and to control its exit from the modulation zone and light valve for formation of a controlled pixel of minimally acceptable optical qualities.
The thicker the films, which were required by all prior designs to realize (through increase of the path-length variable 1 of the classical Faraday effect equation) increased Faraday rotation in the quest for power reduction and switching efficiency, the greater this problem. Lacking practical beam control and insertion for arrays, and even more problematic, lack of control of the propagation of rays through the MO material (including both forward propagating rays as well as management of any reflected rays), is more and more of a problem as the safe cone of insertion becomes narrower and narrower the thicker the film for rays that either continue at the edge of a cone angle of insertion with respect to the film surface or, worse, refract and bend and thus depart the pixel/active zone and pass through the fill between pixels and into other pixels.
It should be noted that while planar (or “superficial”) waveguides, especially rib waveguides, had been employed in the fabrication of planar Faraday-effect devices for telecom and other non-display applications for many years, there had been no solution or proposals for using such planar devices for the purposes of any kind of display array to in-couple or out-couple the light required for the input illumination or output sub-pixel or pixel. And these planar devices, even as Faraday-effect devices per se, had limitations of their own, including feature size, lack of sufficient or any real integration of the device (including all the features and active components required for a Faraday-effect-based light valve), and absence of all of the features and techniques possible for improving the performance of the device, whether implemented discretely or integrated.
So, to summarize, efforts to develop a practical magneto-optic array for display (or, for that matter, other array applications such as for spatial light modulators (or “SLM's”) up to and through the development efforts of Litton, included but were not limited to:
1. High power requirements, as usually associated with conventional understanding of Faraday-effect based devices from that era.
2. Continuous addressing of each pixel required by the system substantially contributed to the unnecessarily high power requirements.
3. Contributing to this was the quality of bulk magneto-optic films available or developed, with Verdet constants too low for practical display applications, at least not without major advancements and solutions addressing all other possible aspects of a Faraday-effect-based light valve, array of such, and display (or SLM) of such.
4. The lower the Verdet constant, the greater the reliance on thicker films.
5. Thicker films become more difficult to saturate and permeate with the imposed B field from a coil structure fabricated on the top surface of the film and array.
6. Magnetic cross talk, which is made worse the thicker the films employed.
7. Optical cross talk, which is made worse the thicker the films employed.
8. Unacceptable fill-factor due to the employment of surface-area hogging superficial “squiggle” coil-structures to address the problem of managing current amplitude. The unacceptable fill-factor between pixels creates the “venetian blind” effect of visible pixel gaps, which substantially degrades the human visual systems perception of an array of pixels as a single, integrated image.
9. No color display solution. In bulk MO films, no MO films had been proposed or fabricated which could both transmit sufficient green and especially blue light while also generating sufficient Faraday rotation to implement a native MO blue or green pixel light-valve. And no other solutions, either employing something other than bulk MO films or native MO switching of blue or green light, had been proposed or fabricated. The best performing iron garnet materials of the time, such as Bi-substituted YIG, perform optimally in near-infrared or infrared, and well in red. But very, very poorly and inefficiently for green light, and essentially nothing (vanishingly small) for blue. This is generally due to the absorption of the shorter wavelengths, especially blue, by the iron or iron oxide in the compositions.
10. No display-size scaling solution for displays larger than those possible with small, high-quality MO films restricted to the size of quality films that could be fabricated. This followed from reliance on continuous, defect-free high quality MO films for design approach.
11. No practical resolution-scaling solution for displays of resolution beyond very small resolutions. This was due to the power requirements and the problems of magnetic domain-management of each sub-pixel in series.
And, with the exception of the color requirement, all of these problems also applied to non-display applications of magneto-optic device arrays, such as SLM's for telecom.
Thus, up until approximately 2001, the status of magneto-optic display development might be summarized thusly:
Limited, at best to: small displays or SLM's of up to perhaps 32×32 or 16×16 resolution, displaying at best crude red (pixel) images, and extremely power-inefficient—to such a degree that even for applications that might have benefited from the most obvious reason to pursue a magneto-optic based display non did.
Why, then, all the effort expended?
Because of the potential—inherent in the demonstrated commercial application, over decades, of Faraday-effect based modulators, rotators, and isolators for telecom and sensing applications—to realize extremely high speed switching—i.e., for displays, that means very, very high frame rates—unbeatably high.
In addition, another lure was the potential to be a simpler and easier to manufacture technology than LCD, plasma, or MEMS.
And finally, much greater thermal robustness and stability. MO materials perform better at high temperatures, for instance.
So, there were some very good reasons to make the effort to realize a practical MO-based display system.
Post-Litton: Two Development Programs that Changed the Field of MO-based Displays and SLM's
To accurately characterize the state of the art of MO-based displays and SLM's, two (“post-Litton”) programs, including one directed by the author of the present disclosure, must be described:
First, while labeling them “post-Litton,” that is only a rough characterization as it only applies to the difference between the approximate start of these two programs, both of which arguably began before 2000, and one definitely beginning in 1990, not long after the start of the Litton program.
The first program, an SLM program for primary application to holographic optical storage disc technology (Optware Corporation), under Prof. Mitsuteru Inoue, addressed one of the major limitations of the prior programs and efforts by proposing a solution to magnetic cross-talk between pixels.
The first 128×128 pixel arrays fabricated in the early 2000's realized a form of magnetic pixel isolation by means of deep-ion-etching of LPE thick films of iron garnet materials. Thus, the air gaps (yielding relatively large gaps or fill-factor) between “pixels” implemented a relatively magnetically impermeable barrier between pixels.
In addition, the devices created by Inoue for Optware's holographic disc system included active-matrix addressing, and other updated features required for addressing the MO-array quickly and with some degree of power reduction.
Fast switching speeds were realized, on the order of 25 ns.
While there were other valuable attributes of this device solution for the SLM application, on its own that major design improvement over Litton did not address the many other problems of realizing a practical MO-based display.
The second program, which entered commercialization phase in the early 2000's after development that began in 1990 and which was based on proposals, including patents issued and pending by the author of the present disclosure plus contributions on materials innovations by other members of the team, yielded the following innovations and improvements, among others:
1. Solutions for optical crosstalk: Optical waveguide control over light path.
2. Solutions for magnetic crosstalk: use of impermeable materials to isolate, and optionally with highly-permeable materials to “pull” field lines “in” towards the pixel.
3. Bi-stable MO/MPC switch for Power Reduction and switching efficiency using Composite magnetic materials structures (also called “exchange-coupled” materials structures) to implement “latching” of pixels, so that pixels may be addressed with a short pulse of current to the field structure of a sub-pixel/pixel instead of a continuous addressing to the pixel.
4. Bi-stable MO/MPC switch for Power Reduction and switching efficiency: Development of latching MO materials and films, by chemical composition, are latchable as individual “bulk” films.
5. Color display: The first practical MO “blue” materials, which demonstrated sufficient transmission for the human visual system in a display system as well as sufficient Faraday rotation for sufficient contrast.
6. Color display: filtering methods, used on conjunction with color-efficient MO materials.
7. Power reduction and switching efficiency: first 1D magneto-photonic crystal devices for display, both multi-layer films and planar gratings structures. Leveraging demonstrated contribution of photonic crystals in the context of non-reciprocal classical Faraday effect, the effective path-length is increased and other enhancements over the “bulk” Verdet constant of any of the MO material layers. Major enhancement of faraday rotation and transmission over bulk materials.
8. Power reduction: Multi-layer coil structures for more efficient MO/MPC film penetration and thus power reduction—top, bottom, and intermediate coil structures, with negligible impact to fill-factor.
9. Power reduction and magnetic cross-talk reduction: Transparent, in-path coil structures for improved field penetration and reduced magnetic cross-talk and no negative impact on fill-factor.
10. Implementation of surface plasmons for potential improvement in device simplification.
11. Implementation of ring-resonators for more compact (reduced feature size), especially for SLM's and chip-platform displays for projectors, etc.
12. Display-size scaling solutions, including through implementing integrated MO-based switch in an optical fiber, as a fiber-device: making larger displays by fiber device arrays, including fabricating the arrays in textile-composite structures and through textile-composite type fabrication and other mechanical fabrication processes.
13. Fully-integrated “3D” MO/MPC switches, planar switches, and fiber switches, for reduced cost of manufacture and greater efficiency, including all “conventional” and new switch elements—composite magnetic materials for domain management and bi-stable switch/latching; polarizer and analyzer (“crossed polarizers”), color filtering, coil-structure, waveguiding, magnetic isolation, etc.
14. Commercialization of cheaper quartz and silicon substrates using lower-temperature film fabrication technologies.
The solutions listed (previously disclosed and developed as devices, materials, and working display systems) by the author of the present disclosure or developed by the teams under his direction, fully addressed all the obstacles to realizing a practical MO/MPC-based display that had existed previously, even after the important work of the Litton and all previous programs. Among the benchmarks set:                Pixel switching speeds less than 15 ns were demonstrated, among other high-performance attributes—approximately 1 million times faster than LCD and 1000 times faster than DMD.        Low-power, bi-stable switches otherwise only realized by lower-image quality B&W display technologies like E-Ink (electrophoretic), fully addressing one of the greatest misconceptions and previous limitations and criticism of the concept of a MO-based display.        Full-color capable with better transmission efficiencies than LCD.        Solid-state crystal devices of much simpler device and manufacturing complexity, and lower cost        Lower cost display-size scaling solution, cheaper than that achieved by LC        Thermally stable and robust, reducing cooling requirements and thus operating cost.        
However, it is the contention of the present disclosure that there are none-the-less further significant improvements possible for MO and MPC related devices, especially configured in arrays both for display and non-display applications (on-chip or spatially-separated). And several improvements of this kind are disclosed herein.
And that furthermore, it is the contention of the present disclosure that MO and MPC devices can contribute to an overall display system more effectively by being designed and optimized for those wavelengths for which best available materials and materials structures are natively best-performing, serving then instead as one method and stage in a de-composed display device pixel-signal processing system.
The reasons for adopting this strategy for the design optimization of MO/MPC devices in display system are explored more generally in some of the incorporated application, and it is pointed out for most if not all signal processing technologies, in which some attribute of a signal (pixel signal or information signal) is modified actively (energized) or passively (un-energized), the physical effect or process involved is materials dependent, requiring certain materials and not others; and for those materials, they are to some degree wavelength dependent.
So, for MO/MPC, as with other photonic or opto-electronic devices such as Mach-Zehnder devices, the physical effect is most effective and/or efficient for some frequencies/wavelengths over others.
In the conception of a display system based on multi-stage pixel-signal processing, this then implies that, whenever possible, all pixel-signal (or signal) processing stages are conducted using “frequencies/wavelengths of convenience.” And that frequencies/wavelengths are modulated (shifted) between those stages to realize optimal (or more appropriate and thus closer to optimal) input frequencies.
As a practical matter, and for the present disclosure, it is observed that MO and MPC materials and structures continue to perform best in red/near-infrared/infrared regime, as is in fact also true of many other signal processing techniques whose function is to encode an optical signal with information (either data signal information or pixel signal information).
This fact continues to hold true even as photonic bandgap structures gain in effectiveness or efficiency moving from 1D to 3D periodic structures, and is also true as novel properties are discovered from fabrication of nano-scale materials of different sizes and/or shapes. This persists as well in the case of certain types of composite meta-materials in which nano-crystals are encapsulated by other structures through materials synthesis, such as through so-called molecular self-assembly employing room and in general relatively low-temperature colloidal solutions. Still, the new properties that may be sought tend to perform best at certain wavelengths over others.
The novel and/or improved properties, such as differing colors at nano-scale vs. bulk for the same chemical formulation, or differing properties due to different geometric/materials structures such as graphene vs. carbon nanotubes, as demonstrated in a range of ongoing materials and nano-materials research and innovation, continue to vary in strength/intensity with wavelength (and current amplitude, fields, etc.)
Fully-synthetic meta-material types composed of combinations of nano-scale antennae and ring-resonators may provide a pathway to greater flatness of response and performance across frequencies, voltages, current amplitude, field strengths, etc, though it may be reasonably hypothesized that there will be variability of performance based on the materials used to form the synthetic structures. And, more importantly, extremely broad-band response will almost certainly be the exception rather than the rule for materials performance going forward.
It is the recognition of this fact, of the wavelength-dependent response of materials for signal processing, that suggests the substantial improvement to the performance of a display system employing magneto-optic modulation (or magneto-photonic modulation) techniques to encode the basic on/off information of the pixel signal, specifically, that a better system, the system of the present disclosure, will result from optimizing an MO/MPC device design for modulation in non-visible near-infrared, followed by subsequent, device optimized pixel-signal optimization steps until the pixel signal exits the display system as an element of an image observed by the human visual system.
It is noted as well that this disclosure, of device optimization according to “wavelengths/frequencies of convenience,” applies in most cases equally as well to non-display device arrays and photonic integrated circuits (PIC's).
As is further disclosed in some of the incorporated application, this observation also applies to a display system employing other best-in-breed signal modulation devices such as Mach-Zehnder interferometer devices, but the specifics of the present disclosure are focused on the details of an hybrid MO/MPC-based display device, working in combination with other pixel-signal processing devices and especially a frequency/wavelength modulation means, to realize a superior MO/MPC based display system.
What is needed is a system and method for re-conceiving the process of capture, distribution, organization, transmission, storage, and presentation to the human visual system or to non-display data array output functionality, in a way that liberates device and system design from compromised functionality of non-optimized operative stages of those processes and instead de-composes the pixel-signal processing and array-signal processing stages into operative stages that permits the optimized function of devices best-suited for each stage, which in practice means designing and operating devices in frequencies for which those devices and processes work most efficiently and then undertaking efficient frequency/wavelength modulation/shifting stages to move back and forth between those “Frequencies of convenience,” with the net effect of further enabling more efficient all-optical signal processing, both local and long-haul. A specific object of the particular and improved systems of the present disclosure are configurations designed around the optimal use and operation of magneto-optic type devices and operations, performing key operative stage(s) in non-visible IR/near-IR frequencies, integrated with best-in-breed frequency/wavelength modulation/shifting means, and together implementing novel all-optical “display over network” and all-optical network migration, compatible with and evolving the next generation of dense wavelength division multiplexing (DWDM)-type networks.